System employing a direct digital synthesiser

ABSTRACT

A DDS based system, such as a radar, includes means for generating a plurality of transmission signals using a DDS, and means for integrating signals derived therefrom, such as received signals. The system further includes means for varying the relative starting phase of the plurality of transmission signals, or adjusting the DDS input clock while maintaining similar primary output frequency characteristics of the transmission signals. The approach has the effect of changing the location of unwanted frequency spurs in each of the transmission signals, and hence the effects of these are decreased in the integration process. An improvement in the sensitivity of the system results. Although primarily suited to radar applications the invention may find utility in other systems such as sonar or lidar systems.

This invention relates to systems that use a Direct Digital Synthesiser(DDS) for signal generation or processing. In particular, it relates tosystems that coherently process signals produced by a DDS, and whichintegrate a plurality of such signals. Such a system may comprise aradar system, typically a CW radar or modulated CW radar.

In a radar system, noise can appear from several different sources. Forexample, thermal noise is produced by materials due to the randommovement of atoms and so has a noise power directly proportional totemperature. Other sources of noise are produced in the radar system'selectronics. Some of these may also be random in nature and willtypically have a Gaussian distribution, while others may be due to, forexample, unwanted or spurious outputs from components or subsystems inthe radar, and may not be random in nature.

Many types of radar system are designed to take a plurality ofmeasurements from a region, and to process the plurality of measurementstogether in some way. This often comprises an integration stage, wheresignal returns received from the region are summed together, generallyto improve signal to noise ratios and hence detection capabilities. Thissummation is generally done coherently, i.e. where both the phase andamplitude of the signal returns are taken into account. Coherentintegration is beneficial in that it gives an improved ability to reducethe effects of some types of noise. Noise of a more random nature willtend to cancel when integrated over a long enough period, due to thevectorial nature of the summation coupled with essentially a randomphase present on each of the returns. Signals e.g. from a target on theother hand will tend to remain after an integration, as the signalreturns all tend to be in phase. Noise of a non-random nature will oftentend to remain following the integration for the same reasons.

Systems such as Frequency Modulated Continuous Wave (FMCW) radars willtypically be arranged to transmit a series of signals into a region andto receive reflections of the signals from targets (be they desiredtargets or clutter). The signals will typically be linear frequencysweeps each having identical properties in terms of their start and stopfrequencies. The radar will integrate the received signals to improvethe signal to noise ratio as discussed above. The number of signals tobe integrated will depend on system parameters such as the time taken toproduce a single frequency sweep, and (for a rotating or electronicallyscanned radar) the dwell time on the target region. Typically a systemmay integrate between 16 and 1024 signals in its processing, and thetime interval over which those signals are generated and processed istermed the integration time.

Radar systems are increasingly using signal generation techniques thatincorporate DDS devices, due to their inherent flexibility and precisecontrol of their output parameters. DDS devices allow complex modulationsignals to be generated simply and repeatably. The outputs of suchdevices typically comprise a wanted signal (termed herein the primaryoutput), but will also comprise other signals, these being artifacts ofthe operation of the DDS. The artifacts comprise unwanted signals atamplitudes generally many orders of magnitude (typically −60 dBc to −80dBc) below the desired signal, and which appear at determinatefrequencies, some of which may be harmonically related to the primaryoutput whereas others may not. These are generally known as spurs. Asthey are typically so small in relation to the wanted signal, they oftendo not pose any problems. For some applications however, the spurs canhave a significant detrimental effect on system performance. One suchapplication is in CW radar, where DDS devices can be used to modulate alocal oscillator (LO) to generate a signal to be transmitted. In suchsystems the spurs present on the DDS output will also modulate the LO,which has the effect of increasing the apparent noise floor of radar,which may mean that smaller targets are much more difficult to detect.

According to a first aspect of the present invention there is provided asystem employing a direct digital synthesiser (DDS), the system beingadapted to use the DDS to provide a modulated signal for transmission,the modulated signal comprising at least a first followed temporally byone or more subsequent signals generated over an integration period, thefirst and subsequent signals having similar primary frequencycharacteristics, each signal having an associated starting phase, thesystem further incorporating an integrator for integrating signalsderived from the first and subsequent modulated signals;

-   -   the derived signals being comprised of intermediate frequency        (IF) signals produced by mixing each signal with a delayed        version of itself;    -   wherein the DDS is provided with at least an input clock source,        an input allowing control of the starting phase for each signal,        and an input for controlling the DDS output frequency;    -   characterised in that the DDS is arranged to generate a primary        output frequency characteristic, the characteristic being the        same for both the first and subsequent signals over the        integration period, wherein the DDS is arranged to have at least        one of the input clock source frequency and the starting phase        changed between production of the first signal and a subsequent        signal.

The limitation that the first and subsequent signals have similarprimary frequency characteristics means that the wanted frequencies atthe output of the DDS and involved in the modulation, including anystart and end frequencies, along with any frequency sweep parameters,are the same for both the first and subsequent signals—it is theseparameters that define the primary output frequency characteristics.Note that frequency spurs, as described below, along with any noisegenerated within the DDS, do not count as primary outputs or primaryfrequency characteristics (as defined herein) of the DDS. Although theinvention has utility when only two signals—the first and an immediatelyfollowing signal—are used, the invention may be used with any suitablenumber >1 of signals. The first and subsequent signals may comprise anysuitable signal, e.g. a frequency swept signal, with the sweep beinglinear, stepped, or having a more complex non-linear arrangement asappropriate for a particular application. The signals may also comprisephase modulated signals. The only criteria being that the primaryoutputs of each of the first and subsequent signals generated within anintegration period are the same.

The invention provides a means to at least ameliorate some of the statedproblems traditionally encountered with DDS devices used in certainenvironments. By using a system according to the present invention, thespurs created by the DDS will have differing characteristics in each ofthe first and subsequent signals. If each of the first and subsequentsignals is arranged to have a different phase, then the spurs will alsoin general have different phases, and will, because of the integrationprocess, not accumulate in the same manner as the wanted signals.

An application of an embodiment of the present invention is in the fieldof FMCW radar systems, where typically the transmitted signals willcomprise a sequence of frequency sweeps. Here, a first frequency sweepcorresponds to the first signal, and subsequent sweeps correspond to thesubsequent signals. These frequency sweeps are generally arranged to belinear sweeps having predefined start and stop frequencies. The start,stop, and sweep characteristics (i.e. sweep duration and frequencycharacteristics) are known herein as the primary frequencycharacteristics. Generally the frequency characteristic comprises alinear frequency sweep, although other such frequency characteristicsmay be used.

The invention makes use of the observation by the inventors that theinitial starting phase of the wanted signal is not relevant in terms ofthe coherent summation process, as it disappears during production ofthe IF signal, as is shown in more detail later.

The system has benefits over the prior art where at least two signals(i.e. the first and a subsequent signal) are generated by the DDS in themanner described. Preferably the DDS is adapted to produce at least 4,such as at least 8, such as at least 16 such as at least 32, such as atleast 64 such as at least 128 such as at least 256 signals within anintegration period, each having similar primary output frequencycharacteristics, and each being derived from a different input clockfrequency or having a differing output phase.

For systems such as FMCW radars the number of signals to be integratedin an integration period (which is typically the radar's dwell time or asub-multiple thereof) will generally be predefined. Advantageously, ifthe number of signals to be integrated is known, then the number ofinput clock frequency changes, or the number of output signal phasechanges is chosen to be equal to the number of signals to be integrated,or to a sub-multiple thereof.

For those embodiments where the output phase of the primary outputsignal is changed for each signal in an integration period, the phasechange may advantageously be arranged to step through one or more full360° cycle (i.e., a full rotation of the unit vector) during theintegration period. E.g. the phase change per step may be 360°/n, wheren is the number of signals to be integrated.

Each phase change applied to the primary output signal may be anythingsuitable. For example, the phase of the output signal may convenientlybe stepped linearly though n 360°/n steps. Alternatively, the outputphase may be chosen in a pseudo random manner. Preferably, during eachintegration period at least 8, such as at least 16, such as at least 32,such as at least 64 different phases are chosen. The advantage of havinga greater number of discrete phases is that the spurs are more likely tobe smeared out during the integration process, i.e. the phases of thespurs are more likely to take on a full range of values, leading to abetter reduction of the effects of the spurs as the signals areintegrated. Clearly there cannot be more phases chosen than there aresignals in an integration period, but if there are fewer chosen thenpreferably an integer number of rotations of the unit vector are chosen.

If the input clock frequency to the DDS is changed between production ofthe first and a subsequent signal then the DDS will in general need tobe adapted to take account of this, if it is to produce a primary outputsignal having similar properties in terms of its output frequency foreach of the first and subsequent signals. This will be done according tothe properties of the particular DDS device used. The manner in which itwill be done will be known to the normally skilled person, asprogramming the DDS device to produce a particular output frequencygiven knowledge of the clock input frequency is a commonplace procedurewith such devices. The input clock may be generated by any suitablemeans. Conveniently the input clock may itself be produced by a secondDDS, as then the clock frequency can be easily changed with suitableaccuracy.

According to a second aspect of the present invention there is provideda method of processing signals in a radar system comprising the stepsof:

-   -   a) using a direct digital synthesiser (DDS) to produce a first        and a subsequent signal as primary outputs, the first and        subsequent signals having similar primary output frequency        characteristics;    -   b) transmitting the first and subsequent signals, or signals        derived therefrom;    -   c) receiving a signal, comprising at least a reflection from one        or more objects, of the transmitted signal;    -   d) mixing the received signal with a portion of the signal being        transmitted to produce an intermediate frequency signal (IF);    -   e) coherently integrating IF signals, or signals derived        therefrom, from the first and subsequent signals;    -   characterised in that, in step a), the DDS is programmed to        change a phase of the primary output between generation of the        first and the subsequent signal.

The phase change amount may be anything convenient. It may be determinedrandomly or pseudo-randomly, or may comprise linear steps, as describedabove in relation to a first aspect of the invention.

The first and subsequent signals may comprise a frequency sweep. Thefrequency sweep may be a linear sweep.

According to a third aspect of the present invention there is provided amethod of processing signals in a radar system comprising the steps of:

-   -   a) using a direct digital synthesiser (DDS) to produce a first        and a subsequent signal as primary outputs, the first and        subsequent signals having similar primary output frequency        characteristics;    -   b) transmitting the first and subsequent signals, or signals        derived therefrom;    -   c) receiving a signal, comprising at least a reflection from one        or more objects, of the transmitted signal;    -   d) mixing the received signal with a portion of the signal being        transmitted to produce an intermediate frequency signal (IF);    -   e) coherently integrating IF signals produced from the first and        subsequent signals;    -   characterised in that, in step a), the DDS has a clock input        provided by a programmable frequency device, and between the        first and the subsequent signal, the programmable frequency        device is arranged to change the clock frequency provided to the        DDS by a predetermined amount, and wherein the DDS is programmed        to compensate for its different frequency input such that that        the subsequent signal has similar primary frequency        characteristics.

The invention will now be described in more detail, by way of exampleonly, with reference to the following Figures, of which:

FIG. 1 shows a block diagram of a typical hardware setup—an FMCW radarin this case—in which an embodiment of present invention may beimplemented;

FIG. 2 shows a graph of an output from a test-bed radar system that usesa DDS in the generation of its transmitted signal, the graph showing aprocessed return from a single transmitted signal;

FIG. 3 shows a graph of an averaged series of outputs from the test bedradar where the DDS is arranged to work according to the prior art, andhence is not adapted to implement the present invention;

FIG. 4 shows a graph of an averaged series of outputs from the test bedradar where the DDS is adapted to implement the present invention;

FIG. 5 shows a block diagram of a typical hardware setup upon which asecond embodiment of the present invention may be implemented; and

FIG. 6 shows a block diagram of third embodiment of the presentinvention, wherein the invention is applied to a heterodyne CW radar.

FIG. 1 shows an FMCW radar system incorporating a DDS, the DDS beingused to provide a modulated output. The system shown is largely similarto that shown in co-pending patent application PCT/GB2008/000306, thewhole contents of which are hereby included by reference. The radarincorporates a local oscillator (LO) 10 operative at 9.2 GHz whichprovides an input to a quadrature up-convert mixer 11. A second input tothe mixer 11 comes from an IF oscillator in the form of a direct digitalsynthesiser (DDS) device 12, in this case implemented using a pair ofAnalog Devices AD9858 DDS chips. As well as providing an input to mixer11, the output of the LO 10 also feeds a first frequency divider 13,which in turn drives a second frequency divider 14. An output from thefirst frequency divider 13 is used as a reference clock source for thedirect digital synthesiser 12. The second frequency divider 14 providesa clock reference source to a controller 15, which may be amicrocontroller, which has controlling outputs connected to the DDS andto analogue to digital converters (ADC) 16, 16′ that are used todigitise incoming signals reflected from targets and other objects. Thecontroller 15 also provides synchronisation data to signal processingmeans 21.

The radar has a receiver chain comprising a low noise amplifier 17, amixer 18 having both In phase (I) and Quadrature (Q) outputs coupled toa pair of IF amplifiers 19, 19′, then to a pair of Nyquist filters 20,20′, defining a pair of channels. The outputs of the Nyquist filters 20,20′ feed ADCs 16, 16′ each of which provides digital signals to signalprocessing means 21. Mixer 18 has a second input taken from the signalto be transmitted.

Transmit mixer 11 is a quadrature up-convert mixer fed at the IF inputwith both an I and Q input from DDS 12.

In operation, the LO 10 produces a 9.2 GHz LO output which feeds oneinput of mixer 11. The DDS is clocked by a reference clock signalderived from the LO output, but divided in frequency from it by aquotient of 10. The output frequency of the DDS 12 is determined by thisclock in combination with the input from the controller 15. The clockinput to the controller is taken from the frequency divider 14 having adivision quotient of 50, which is itself supplied from frequency divider13. Thus the clock frequency supplying the ADCs is 18 MHz. Thecontroller 15 contains logic that triggers the DDS 12 to start itsfrequency sweep, causing the DDS 12 output to ramp linearly between 200MHz and 250 MHz in a repetitive fashion. This output frequency is mixedwith an output of the STALO 10 in mixer 11, to produce the output signalof the radar, of 9.4 GHz to 9.45 GHz. As both the DDS 12 and thecontroller 15 are locked, via dividers 13 and 14, to the STALO 10, theoutput signals of the controller 15 and the DDS 12 are all coherent withthe STALO 10 output. The controller 15 may also be used to reprogram theDDS 12 to change its frequency sweep parameters should such frequencyagility be desired. For example, subsequent integration periods may usesignals having differing primary frequency characteristics

The controller 15 is also adapted to communicate with the DDS 12 so asto be capable of changing the starting phase at the output of the DDS 12for each sweep.

Received signals comprising, amongst other things, reflections fromtargets enter the system via a receive antenna (not shown), and areamplified in low noise amplifier 17. The amplified signal is then mixedwith a signal simultaneously being transmitted by the transmitter, bysplitting off some of the energy in the final stages of the transmitpath, using directional coupler 22. Thus in this fashion an IF signal isproduced that comprises a signal mixed with a delayed version of itself,the delay being created by transmission and subsequent reception ofsignals reflected from targets within a region.

The output of mixer 18 is an I-Q pair comprising the differencefrequency between the received signal and the signal simultaneouslybeing transmitted. The signals are amplified in amplifiers 19, 19′,filtered in low pass filters 20, 20′ before being digitised usinganalogue to digital converters 16, 16′. The digitisers are driven by aclock signal from the controller 15, which as described above is itselfdriven from a clock derived from the STALO 10.

The resulting signals are then processed in processor 21 by calculationof a discrete Fourier transform (which may be calculated using the FastFourier Transform (FFT) algorithm) of each digitised signal, andcoherently integrating the output of the Fourier transformed signalsreceived within an integration period to boost signal to noise levels.The integration period may be chosen dependent upon the application ofthe radar, and upon other parameters such as dwell time, signal sweeptime etc. The integration period may be arranged to vary depending uponthe particular target type being detected. An embodiment of the presentinvention designed for detection of small objects is adapted to produce16 signals, each comprising a frequency sweep having identical primaryfrequency characteristics, within an integration period.

By suitable control of the DDS 12 the arrangement of FIG. 1 may be usedto implement an FMCW radar of the prior art, or may be adapted toimplement an embodiment of the present invention. When adapted toimplement an embodiment of the present invention the controller 15 isadapted to reprogram the DDS so that for each of the first andsubsequent signals generated within an integration period the startingphase of the signal differs as described herein.

FIG. 2 shows a graph of a single sweep from the radar system as shown inFIG. 1. As only a single sweep is shown, it is not apparent from thegraph whether the radar is set up as an embodiment of the presentinvention (e.g. adapted to provide differing phases for differingsweeps), or whether the radar is adapted to maintain a fixed startingphase for each sweep as known from the prior art, as no integration hasyet taken place. A first trace 25 shows a return of amplitude againstrange for a single frequency sweep, i.e. with no averaging of any kindapplied. It therefore represents information from the output of theradar following an FFT processing step. The sweep was directed at ascene containing a large, 1000 square meter radar cross section (RCS)target located at a range of around 250 m, which corresponded to rangecell 617. The large amplitude return from this can be clearly seen.Other, smaller, targets can be seen at longer ranges. However, the noiselevels in the trace 25 are relatively high. These high noise levels areproduced by phase noise in the system, a significant amount of which isproduced by spurs on the output of the DDS. These spurs may typically beat levels of −60 dB compared to the primary output. At these low levelsit has been generally understood by the normally skilled person thattheir contribution to the total system phase noise is minor. It has beenfound however that this is not the case. When large, static objects arepresent in the region, such as the 1000 square meter RCS target shown inFIG. 2, the system phase noise produces the degraded noise floor asshown. Over a single trace, the phase noise is therefore seen to producea significant degradation in the radar performance.

Superimposed on the graph is a second trace 26 showing the thermal noisefloor of the radar. Trace 26 therefore gives an indication of the sortof levels that should be attainable for a realistic system noise floorwhen many returns are averaged. This second trace 26 was produced bycovering the receiver antenna to prevent any external signals fromentering the radar, and non-coherently integrating the resultingsignals, which are due to internal system noise.

FIG. 3 shows a graph again showing the output of the test-bed radar.Trace 30 shows the result of averaging 256 consecutive sweep signals,each one having similar frequency characteristics, and each havingsimilar starting phases. It therefore represents a signal generated andprocessed according to the prior art. Prior to the present invention,one would expect the noise floor to be reduced, due to the averaging, by10 log 256 dB if the noise were random in nature, as explained above.However, it can be seen that noise levels remain largely unchanged,indicating that the noise—largely consisting of phase noise—is coherentand so does not decay with averaging.

The second trace 31 is shown for reference and is identical to, trace 26of FIG. 2.

FIG. 4 shows a graph again showing the output of the test-bed radar.Trace 40 is an output of the radar again comprising the mean of 256consecutive sweep signals each having similar frequency characteristics.However, this time the controller was adapted to generate each sweepsignal with a different starting phase. The phase increment chosen was360°/64, or 5.625°. This means that over the 256 consecutive sweeps eachphase increment was used four times, and the unit vector of the startingfrequency of the primary output (i.e. a notional unit vector having aphase equal to the phase of the starting frequency) has had fourcomplete rotations.

Second trace 41 is identical to trace 26 of FIG. 2, and is reproduced toshow comparative improvements of the invention over the prior art.

A significant improvement on the previous graph can be seen, with thenoise level being around 20 dB lower, with no change in the level of thetarget at range cell 617. This is close to the theoretical 24 dB dropthat would be observed if the noise were truly Gaussian random noise.Other targets in cells between 1000 and 3500 may also be seen that werepreviously obscured by noise. The vectorial summation in the integratorhas the effect of reducing the noise levels caused by the spurs. As thephases of radar returns from targets following the mixing process isunaffected by the step in phase of the transmitted signals, theintegration results in an increase in the level of signalsrepresentative of target returns. Note that the trace 40 sometimes dipsbelow the averaged system noise trace 41. It is able to do this becausetrace 40 is a coherently averaged signal whereas trace 41 isincoherently averaged, as explained above.

An analysis of why changing the starting phase of the primary output ofthe DDS according to the present invention provides a benefit whenimplemented in an FMCW radar is now presented.

The phase of a linear frequency ramp waveform transmitted by an FMCWradar can be described by equation 1.

$\begin{matrix}{\phi = {{\left( \frac{\pi\;\Delta\; F}{T} \right)t^{2}} + {2\pi\; f_{0}t} + \phi_{0}}} & {{Equation}\mspace{14mu} 1}\end{matrix}$Where;φ is the phase of the transmitted signalΔF is the frequency deviation of the frequency sweepT is the duration of the sweept is timef₀ is the start frequencyφ₀ is the start phase of the sweep

In the radar receiver the signal currently being transmitted is mixedwith the received signal, i.e. effectively with a delayed version ofitself. Therefore, given a transmitted signal according to equation 2:

$\begin{matrix}{s_{1} = {\cos\left( {{\left( \frac{\pi\;\Delta\; F}{T} \right)t_{1}^{2}} + {2\pi\; f_{0}t_{1}} + \phi_{0}} \right)}} & {{Equation}\mspace{14mu} 2}\end{matrix}$

The received signal will be according to equation 3 (ignoring amplitudelevels);

$\begin{matrix}{s_{2} = {\cos\left( {{\left( \frac{\pi\;\Delta\; F}{T} \right)t_{2}^{2}} + {2\pi\; f_{0}t_{2}} + \phi_{0}} \right)}} & {{Equation}\mspace{14mu} 3}\end{matrix}$

The FMCW radar typically mixes (i.e. multiplies together) the receivedsignal with that signal currently being transmitted.

the resultant component after mixing and low pass filtering is;

$\begin{matrix}{{{\cos\left( \frac{\pi\;\Delta\; F}{T} \right)}\left( {t_{1}^{2} - t_{2}^{2}} \right)} + {2\pi\;{f_{0}\left( {t_{1} - t_{2}} \right)}}} & {{Equation}\mspace{14mu} 4}\end{matrix}$

This shows that the phase of the received IF signal, corresponding tothe radar target, is independent of the start phase of the frequencysweep, φ₀. Therefore, as long as the start frequency, f₀, of the sweepis maintained, the starting phase can be varied from sweep to sweepwithout affecting the coherency of the radar.

It is believed that the spurs present on the output of the DDS arepredominantly due to the Digital to Analogue (DAC) within the DDS IC,coupled with the limited resolution of the sine wave look up tablewithin the DDS. By changing the start phase of the sweep, the DAC willbe exercised through different DAC quantisation levels and the phase ofeach spur will also change. As the phase of the spurs are different fromsweep to sweep, the coherent averaging process leads to a reduction inthe effect of the spur on system performance.

FIG. 5 shows a second embodiment of the present invention. As thereceive architecture of this embodiment is identical to that describedin relation to FIG. 1, it will not be described further. On the transmitside, LO 50 provides an output to quadrature up-convert mixer 51. LO 50also provides an input to frequency divider 52, which provides an outputat 1/10^(th) of its input frequency. An output of divider 52 drives aclock input to a first DDS device 53. The first DDS device 53 providesan output to a clock input of a second DDS device 54. The second DDSdevice 54 produces an I and Q IF signal to quadrature up-convert mixer51. Frequency divider 52 also feeds a second frequency divider 55 whichproduces a clock signal for controller 56. Controller 56 providessynchronisation signals for the DDS devices and also for the signalprocessor in the receive chain. Controller 56 is also arranged to beable to change primary frequency characteristics of both DDS devices.

The purpose of the first DDS is to provide a clock frequency that isaccurately controllable within a small range of frequencies. A DDS istherefore ideal for such a task, although in practice any convenientmeans of generating a variable but predetermined and accurate clockfrequency may be used.

In operation, the second DDS 54 is arranged to produce an IF signalcomprising a repetitive, linearly frequency swept signal starting at 100MHz and finishing at 150 MHz. To do this, controller 56 programs thesecond DDS 54 with, amongst other things, information regarding theinput clock frequency. The input clock frequency of the second DDS isaround 400 MHz, although as explained below, it is changed at regularintervals. Controller 56 programs the first DDS 53 to provide theexpected input clock frequency to the second DDS 54. The repetitivefrequency sweeps generated by second DDS 54 comprise a first sweepsignal, followed temporally by at least one subsequent sweep signal,each having the same primary output frequency characteristics. However,between the generation of the first and a subsequent signal, thecontroller 56 is arranged to reprogram the first DDS 53 to provide aslightly different clock frequency as its output. The change maytypically be, say, from 400 MHz to 401 MHz. The second DDS 54 isprogrammed with information regarding its new input frequency, so thatit is able to maintain the same primary output frequencycharacteristics. This may occur for each sweep signal generated withinan integration period, although it is not necessary to change the inputclock frequency of the second DDS (and hence also reprogram it toprovide the same primary output frequency) for every sweep generatedwithin an integration period. As is the case with the embodimentdescribed in relation to FIG. 1, the DDS parameters (input frequency ofthe second DDS for this embodiment, and output phase of the earlierembodiment) may be changed at least once, and up to n times, where n isthe number of signal sweeps provided within an integration period. Thegreater number of changes that occur however within an integrationperiod, the more the effect of the spurs on the system's apparent noisefloor is reduced.

This is because as the parameters within the second DDS 54 changebetween each of the first and subsequent signals, then the spursproduced by the second DDS will also have different properties. Theywill tend to appear at different frequencies for each of the first andsubsequent signals. The integration of the signals in the receive signalprocessing system 57 means that contributions due to the spurs will becoherently integrated and so will tend to reduce as compared to thecontributions due to the primary output signals.

The input clock change to the second DDS 54 may be anything suitablethat has the effect of producing differing spur characteristics at itsoutput. This may vary dependent upon a particular DDS device used.

FIG. 6 shows a third embodiment of the present invention. This is a CWradar broadly similar to the embodiment described in FIG. 1, with theexception that it is a heterodyne system. Heterodyne systems arecommonplace in radar architectures, due to their often superior noiseperformance, and convenient filtering requirements. STALO 60, the systemfrequency reference, generates a stable 7742 MHz output. This output isprovided to quadrature up-convert mixer 61 and to frequency divider 62.Divider 62 is a “divide by 8” divider (N=8), and so provides a 967.75MHz output to the clock input of a first DDS 63 and to a second divider64. The first DDS 63 has quadrature outputs and is arranged to produce arepetitively a swept frequency signal of between 100 MHz and 150 MHz atits output. The quadrature outputs provide a second input to mixer 61.

The output of mixer 61 is input to a frequency mixer 65 that in thisembodiment multiplies the frequency by factor 12. Thus its output is aswept frequency waveform from approximately 94100 MHz to 94700 MHz. Thissignal is input to a second quadrature up-convert mixer 66. A second DDS67, that again takes its clock input from the output of divider 62, isarranged to provide quadrature outputs at a fixed 400 MHz outputfrequency. These outputs are provided as a second input to mixer 66. Theoutput of mixer 66, at around 95.5 GHz to 96.1 GHz, provides thetransmit signal for the radar (although additional amplification etc maybe provided if necessary). A person having ordinary skill in the artwould appreciate that a frequency divider could be used in place ofsecond DDS 67 (with consequent changes to the LO frequency), but that aDDS provides agility to the system for, for example, frequency hopping.

The receiver comprises low noise amplifier 68, the output of whichdrives an input of 1^(st) LO mixer 69. A second input to mixer 69 comesfrom the output of frequency multiplier 65. The first LO is thedifference in frequency between these two, and so is 400 MHz+a targetbeat frequency, which, as would be understood by someone of ordinaryskill in the art, will be dependent upon target range. The 1^(st) LO isfiltered in band pass filter 70, and then provided to an input of a2^(nd) LO mixer 71. A second input to the 2^(nd) LO mixer 71 is takenfrom the second DDS 67, running at 400 MHz. The output of the 2^(nd) LOmixer 71 is thus the target beat frequency. This signal is processed insignal processor 73 in known fashion, by, among other steps,amplification of the output of mixer 71 in amplifier 72 and integrationover an integration period of the signals to improve signal to noise, aspreviously explained. Controller 74 is arranged to adjust the settingsof DDS 63 so that, for the signals generated within a single integrationperiod, the starting phase is adjusted a plurality of times, asdescribed above. Thus the integration process will also ameliorate theeffects of frequency spurs from the DDS 63, by reducing them relative tothe primary output signals from DDS 63.

The scope of the present disclosure includes any novel feature orcombination of features disclosed therein either explicitly orimplicitly or any generalisation thereof irrespective of whether or notit relates to the claimed invention or mitigates any or all of theproblems addressed by the present invention. The applicant hereby givesnotice that new claims may be formulated to such features during theprosecution of this application or of any such further applicationderived there-from. In particular, with reference to the appendedclaims, features from dependent claims may be combined with those of theindependent claims and features from respective independent claims maybe combined in any appropriate manner and not merely in the specificcombinations enumerated in the claims.

The invention claimed is:
 1. A system employing a direct digitalsynthesiser (DDS), the system being adapted to use the DDS to provide amodulated signal for transmission, the modulated signal comprising atleast a first followed temporally by one or more subsequent signalsgenerated over an integration period, the first and subsequent signalshaving similar primary frequency characteristics, each signal having anassociated starting phase, the system further incorporating anintegrator for integrating signals derived from the first and subsequentmodulated signals; the derived signals being comprised of intermediatefrequency (IF) signals produced by mixing each signal with a delayedversion of itself; wherein the DDS is provided with at least an inputclock source, an input allowing control of the starting phase for eachsignal, and an input for controlling the DDS output frequency; whereinthe DDS is arranged to generate a primary output frequencycharacteristic, the characteristic being the same for both the first andsubsequent signals over the integration period, wherein the DDS isarranged to have at least one of the input clock source frequency andthe starting phase changed between production of the first signal and asubsequent signal.
 2. A system as claimed in claim 1 wherein the DDS isarranged to generate at least 4 different phases over an integrationperiod.
 3. A system as claimed in claim 2 wherein the phases are chosensuch that, during an integration period, a unit phasor makes a wholenumber of rotations around a unit circle.
 4. A system as claimed inclaim 2 wherein the phases are changed in a linear manner.
 5. A systemas claimed in claim 2 wherein the phases are changed in a pseudo-randommanner.
 6. A system as claimed in claim 1 wherein the first andsubsequent signals comprise linear frequency sweeps.
 7. A system asclaimed in claim 1 wherein the input clock source for the DDS is itselfprovided by a second DDS.
 8. A system as claimed in claim 1 wherein atleast 4 signals are generated in a single integration period, whereinthe number of signals is at least equal to the number of phases.
 9. Asystem as claimed in claim 1 wherein the mixing of each signal with adelayed version of itself occurs in a receiver, with the delay beingproduced by signal flight time from a transmit antenna, reflection froma target, and subsequent reception at the receiver.
 10. A radarincorporating a system as claimed in claim
 1. 11. A radar as claimed inclaim 10 wherein the radar is a frequency modulated continuous waveradar.
 12. A method of processing signals in a radar system comprisingthe steps of: a) using a direct digital synthesiser (DDS) to produce afirst and a subsequent signal as primary outputs, the first andsubsequent signals having similar primary output frequencycharacteristics; b) transmitting the first and subsequent signals, orsignals derived therefrom; c) receiving a signal, comprising at least areflection from one or more objects, of the transmitted signal; d)mixing the received signal with a portion of the signal beingtransmitted to produce an intermediate frequency signal (IF); e)coherently integrating IF signals produced from the first and subsequentsignals; wherein, in step a), the DDS is programmed to change a phase ofthe primary output between generation of the first and the subsequentsignal.
 13. A method as claimed in claim 12 wherein step e) is adaptedto coherently integrate at least 4 signals, the total transmission timeof the integrated signals defining an integration period.
 14. A methodas claimed in claim 13 wherein, during the integration period, the DDSchanges the phase of its primary output n, or a sub-multiple of n times,where n is the number of signals within an integration period.
 15. Amethod as claimed in claim 12 wherein each signal comprises a linearfrequency sweep.
 16. A method of processing signals in a radar systemcomprising the steps of: a) using a direct digital synthesiser (DDS) toproduce a first and a subsequent signal as primary outputs, the firstand subsequent signals having similar primary output frequencycharacteristics; b) transmitting the first and subsequent signals, orsignals derived therefrom; c) receiving a signal, comprising at least areflection from one or more objects, of the transmitted signal; d)mixing the received signal with a portion of the signal beingtransmitted to produce an intermediate frequency signal (IF); e)coherently integrating IF signals produced from the first and subsequentsignals; wherein, in step a), the DDS has a clock input provided by aprogrammable frequency device, and between the first and the subsequentsignal, the programmable frequency device is arranged to change theclock frequency provided to the DDS by a predetermined amount, andwherein the DDS is programmed to compensate for its different frequencyinput such that that the subsequent signal has similar primary frequencycharacteristics.